Method and apparatus for zone melting



if l April 16, 1957 METHOD AND APPARATUS FOR ZONE MELTING Filed Aug. 25, 1953 /ll/l f llllllfllllI/lllllfll l, ffrzlff fr',

I NI/ E NTOR.

R nBEH-r Y JENSEN TTOR NE Y R. v. JENSEN n 2,789,039

United States Patent O Robert V. Jensen, Hightstwn, N. I., assigner to Radio Cdrpoalion of America, .11" c'trperation` .of 'Delaware Application Alig'u'stl25; v 1953; SerialNo; 376,404f

4 Claims. (Ci. ISV-30H.

This invention relate'sf to? methods' and apparatus for Z''Iie `meltin`g` and more particularly to irripiv'edfmethods andapparatusrv for gro'wngsingle' crystals of materials such as germanium by azone melting process.

The principles of zonemelting'are well known.- See, for example, an article: entitled VPrinciples of Zone MeltingVby W. G. Pfann', in the Journal of Metals foi-July 1952, page 747. In zone melting a' relatively narrow molten zone is caused t'otraverse the -length of an elongatedcharge of anfalloy--o'r arnetal. Thep'rocess has been 'found advantageous in' purifying metals and is' especially useful in purifyin`gf-semiconductor materials such as' germanium; Theproce's's is' also adaptable to'growing-a single crystal.

ln'growing' a single crystal ingot by the zone melting technique' the" shape of thev advancing interfacebetween the molten zone and th'egrowing crystal is important. Thisfinterface, in order'to' promote single crystal growth of the ingot, isfpreferablymade convex` with respect to tlie growing crystal. Crystal growth tends tolprogress in a direction normalto the interface. A convex shaped interface, therefore, provides a freezing' frontsuch that undesired nonuniform crystal growths tend to-extend toward the outer edges of' the ingot and notto continue along the length of the ingot. A concave interface, on the other hand; permits the growth of undesired crystal structures from'fthe outer edges of the crystal toward the center causing non-uniformity of th'e growing crystal.

Previous methods of growing single crystals by the zonemelting process comprise the use of an insulating crucible or boat, usually of silica. The walls of4 an insulating Crucible have a greater thermal resistance than does a metallic` charge in the Crucible. Therefore, the charge cools by conductionof heat down its length rather than by conduction towards itsV outer edges. Thus, central portionsy of thev ingot'lare made to freeze before the outer edges to provide a convex interface.

Insulating crucibles, however, are subject to certain disadvantages which are overcome'by the instant invention. They impede `the ilow of heat from a furnace to the chargein the Crucible and tend to Vlengthenthe molten zone. cult to form into accurate shapes. Quartz, for example, cannot be machined, nor can it-be readily formed into a Crucible; having a sharply rectangular cross-sectional shape.

Accordingly it is'anobject of the invention to provide improved apparatus for zone melting.

Another object is to provideimproved methods of zone melting utilizing a thermally conductive Crucible.

Another object is to 'provide improved methods of zone melting utilizing a` Crucible ofaconductive, relatively soft, machinable material.

A' further object isto provide an improved'method of zone melting suitable for growing single crystalsof materials such as germanium.

According to theinvention singleecrystal growth is conducted' in a" zone melting furnace. A desired convex interface between the growing crystal and the `molten They are usually brittle, easily broken, and dii- 3 ice 2 zone" is provided b'yf utilizngA af 'conductiver` crc'iblefl and heating the' Wallsofl thecrucibletpreferbly'by p'afssiig an electric current through them. Thus the out edges off'a charge-inf thec'rucible are"-heatedfbythe` crlcible walls in-l addition tof the zoneiffurnac'ef heatfsfthal the central-hportionsfof the-charge freeze'istftoi produce a desired convex interface-:w Y y.

The 'invention wi11abe-deseri5ed iin greateefdetali with reference to the drawing of which the singleg fis aVVV partially? schematic, crosslsectional'f plriview ofi a paratusf aecordingtothe: invention?y v The" zone furnace'- may'. be heated by? any convenient meanslsuch asfthe electric'resi'stanchet" gielements I8 connectedl to any convenientpower" so'u' /c'ef' (noty shown). The? vfurnace is adapted to" travel alo'rxgfth'el' length i of the crucible from' one endltol the'iother ata controlled rate. Any` convenient `means may be" p'rovi'ded` controllably to propel the furnace' along tlie tube." Forexampleythe furnace may be supportedfbyth'e'two' brackets 24 a\"n'd"2'5 which rest on" the rail `27 andY the screw'- 2'6 frespeetively. Thev screw maybe rotated b'yf'themotor` 28 -to propellthe furnace in a desired?- dir'ecti'o'n' a't aI controlle' speed. Alternatively, the" furnace maybe'heldetatioitiaiyy aiidthe Crucible may be adapted toN progress'tlrou`g'h`thefrnaee. Electrical leads 20 are' connectedtofopposit`'erfdsSofithe crucibleand vto tbe terminals of tli'ebattery 2'2"," oi'other powerl source; .to induce allio-Wtoflelectrieicurrent vtl'1`rtii'1'gh th'ewvalls of` the cruciblei Inf` atypical apparatus?the"4 cr'cibl; @may bei' about 24" long-fandr 1" diametergrfor."example, and? meegermani'um ingot'rnay'wei'ghf-abou'tl'l! kilo'grn.- Thel size of the Jseedcrystal 1 is noticri'tical but fit." shouldbeflige enough?V so that" alportionofl it may? b'ef nieltedf"vi'i'tlot meltingtheentire crystal. In operation, aprotec'tive gas such as hydrogen or an inert gas of the zero column or the periodic tabie is maintainedwvithirn the tube.

Suflicientelectric power'is applic ofthe carbon crucible' toheat the crucible'antl fits'contents'ito'a!temperature about 200 to 300 C. belowftl'le meltingpointofthe-germanium.

The zone furnace is'move'dclose'toltlel seed crystal end ofthecrucible and heated.l A'sisoonaslthe zone furnace heatlhas' melted" al zone'of the in'gota-bo'uti 1" long, the furnaceis moved-'slowly towa'rtlth'e seedicrystal until the molten zone includes a portion of thse'etl crystal. The furnace is lthen'drit'venback aWy-'fromf the seed crystal toward /the oppositeend loffth-i'crucihle at a rate of approximately l3 'mn1".-permn1te;`

The temperature ofthe-'moltenizonel'is nt'icr-itical. It shouldl besub'stantially hilghenthafn the* melting "p'ont of the ingot material in order toinsr'e-icornplte meltfih'g'lof the entire mass Swithinthemolten zonea- If'- the "molten zonefis maintained at-lai"temperaturefionly slightly" above the melting point, relatively small solid crystallites of the .but will grow in a polycrystalline form.

material may remain in the molten zone without melting. Ifhese crystallites may provide additional nucleating centers as the zone progresses and interfere with the grow-th of the large single crystal. Generally, any temperature at least about 50 C. above the melting point of the material of the ingot is sucient to insure complete melting of .the zone.

As the furnace is driven along the length of the crucible,

a continually changing portion of the ingot is melted and ,refrozen. In v refreezing, the material forms a substantially single crystal structure whose orientation is determined by the Seed crystal initially placed at one end of the ,crucible y .The rate of travel of the furnace is only slightly variable according to the temperature at which the molten zone is maintained. In general, for reasons of economy, it is .desirablelto move the furnace at approximately the maximum -crystal growth rate. If the furnace is moved too rapidly, the freezing ingot will not form a single crystal In growing a single crystal of germanium, for instance, a speed of about l to 3 mm. per minute gives satisfactory results when the molten zone is maintained about 50 C. above the melting -point of germanium. The maximum growing speed also .varies according to the orientation of the crystal growth.

Single crystal growth is promoted by the shape of the liquid-solid interface as explained heretofore. For example, discontinuities in the walls of the crucible may provide undesired nucleating centers for the growth of disoriented crystals. Such crystals tend to grow in a direction normal to the crucible walls. While still relatively small, therefore, they meet and stop growing at the advancing boundary of the principal crystal. A convex interface is provided by the resistance heating of the walls of the crucible, which permits freezing to take place primarily by cooling from the ends of the crucible. Thus,

-heat travels from the outer edges of the newly formed c rystal toward the center. The outer edges thus are maintained at a higher temperature than the central portion of the crystal so that the central portion is made to freeze before the outer por-tion.

When the furnace has traversed the entire length of the crucible, the entire ingot has been converted to a single crystal and the process is complete.

An important feature of the invention is the relatively uniform Aheating of a charge along its entire length simultaneously with additional heating of a zone portion of the charge. The uniform heating may be conveniently provided by passing an electric current through an electrically conductive crucible.

material to be melted is not inert with respect to the material of the crucible an inert liner or boat may be provided chemically to insulate the crucible from the charge. For A example, in melting silicon, which is adversely affected by carbon, a silica liner may be provided in a carbon crucible.

It should be understood that the practice of the invention is not limited to heating a conductive crucible by inducing an electric current in it. The instant invention contemplates providing relatively uniform heating of a crucible by any known means such as a gas tired ame or an electric resistance element insulated from the crucible. Such heating is utilized to provide heating of peripheral portions of the ingot so that cooling of the ingot will occur in an outward direction from the central portions thereof.

The method of heating thezone furnace is not critical.

Electric resistance elements such as bars of silicon car- Crucibles made of mate- Vrials such as carbon or tungsten may be utilized. If the hydrogen or an inert gas to prevent oxidation of the heated metal and to minimize the introduction of impurities into the metal. The presence of a special atmosphere is desirable in many zone melting applications. However, the provision of a protective atmosphere is not an essential part of the instant invention and may be omitted in certain instances such as when zone melting chemically stable salts or oxides.

The practice of the instant invention is not limited to the particular materials described heretofore. It is equally applicable to zone melting of other materials, su-ch as metals generally and salts. Neither is the practice of the invention limited to any particular type of zone furnace. Any known means of providing a relatively narrow moving zone of relatively high heat may be utilized.

What is claimed is:

1. A method of zone melting comprising heating an elongated vessel containing a charge of material substantially uniform-ly along the length of said vessel to a temperature below the melting point of said material, and

raising the temperature of a progressively longitudinally changing portion of said charge substantially above its melting point whereby there is produced a convex interface with respect to said molten zone between the molten zone and the adjacent solid matter.

2. A method of growing a single crystal of a material by zone melting comprising placing a seed crystal of said material at one end of an elongated vesseLplaoing a polycrystalline mass of said material in said vessel adjacent said seed crystal, heating said vessel substantially uniformly along its length to an elevated temperature below the melting point of said material, heating a portion of said material including a portion of said seed crystal to a temperature above said melting point thereby to form a molten zone of said material whereby there is produced a convex interface with respect to said molten zone between the molten zone and the adjacent solid matter, and causing said molten zone to travel along the length of said mass thereby to form a single crystal of said material.

3. Zone melting apparatus comprising an elongated vessel of carbon adapted to hold a molten charge, heating means comprising means for inducing an electric current through the longitudinal walls of said vessel to thereby heat said vessel substantially uniformly along the length of a charge held therein, and means for raising the temperature of a progressively longitudinally changing transverse portion of said charge above the temperature of other portions thereof.

4. Zone-melting apparatus according to claim 3 in which said means for raising the temperature of a portion of said charge comprises a furnace shorter than and substantially surrounding a portion of said vessel, and means for moving said furnace with respect to said vessel along the length of said vessel.

References Cited in the file of this patent UNITED STATES PATENTS Bridgman Feb. 24, 1931 OTHER REFERENCES Andrade et al.: Glide in Metal Single Criptols in Procedures of the Physical Society, 1937, vol. 49, page 152 to 158 inclusive.

Bridgman, article, Certain Physical Properties of Singie Crystals of Tungsten, Etc. in Proceedings of the American Academy of Arts and Scienees; June-Dec. 1925, page 307.

Holden, article, Preparation of Metal Single Crystals in 1949 Preprint, No. 35, of the American Society for Metals; pages 14 and l5.

Chen et al.: article, Growth of Molybdenum Single Crystals in Journal of Metals, vol. 3, Issue 6; June 1951; pages 461 to 464.

Pfann, article, Principles of Zone-Melting in Jourual o f Metals, vol. 4; uly 1952, pages 747 to 753. 

1. A METHOD OF ZONE MELTING COMPRISING HEATING AN ELONGATED VESSEL CONTAINING A CHARGE OF MATERIAL SUBSTANTIALLY INIFORMLY ALONG THE LENGTH OF SAID VESSEL TO A TEMPERATURE BELOW THE MELTING POINT OF SAID MATERIAL, AND RAISING THE TEMPERATURE OF A PROGRESSIVELY LONGITUDINALLY CHANGING PORTION OF SAID CHARGE SUBSTANTIALLY ABOVE ITS MELTING POINT WHEREBY THERE IS PRODUCED A CONVEX INTERFACE WITH RESPECT TO SAID MOLTEN ZONE BETWEEN THE MOLTEN ZONE AND THE ADJACENT SOLID MATTER.
 3. ZONE MELTING APPARATUS COMPRISING AN ELONGATED VESSEL OF CARBON ADAPTED TO HOLD A MOLTEN CHARGE, HEATING MEANS COMPRISING MEANS FOR INDUCING AN ELECTRIC CURRENT THROUGH THE LONGITUDINAL WALLS OF SAID VESSEL TO THEREBY HEAT SAID VESSEL SUBSTANTIALLY UNIFORMLY ALONG THE LENGTH OF A CHARGE HELD THEREIN, AND MEANS FOR RAISING THE TEMPERATURE OF A PROGRESSIVELY LONGITUDINALLY CHANGING TRANSVERSE PORTION OF SAID CHARGE ABOVE THE TEMPERATURE OF OTHER PORTIONS THEREOF. 